Sago Pondweed (Potamogeton pectinatus L.):A Literature Review

Classification and Distribution

The valid scientific name for sago is Potamogeton pectinatus, assigned
by Linnaeus in his Species Plantarum of 1753. The name Potamogeton
is derived from the Greek for "river neighbor," and the specific epithet pectinatus
("comb-like") derives from the closely set insertion of the plant's leaves.
The synonyms P. interruptus Kit., P. latifolius J. Robbins, P.
flabellatus Bab., and P. columbianus Suksdorf have been used in North
American botanical texts. Many other synonyms have used in Europe. Two modern
treatments, Kartesz and Kartesz (1980) and United States Department of Agriculture
(1982), recognize 40 and 35 North American species of Potamogeton, respectively,
and place the genus in the family Potamogetonaceae. Earlier, the genus had variously
been placed in the families Zosteraceae and Najadaceae (Fernald 1950). There
are about 100 species of Potamogeton world-wide (Kadono 1982). Sago flowers
and leaves are simple and anatomically reduced, compared to those of other family
members (Sculthorpe 1967). Sago was one of the first Potamogetons to
be described. An illustration of "fennel-leaved water milfoile" is easily recognized
as sago in the ancient herbal of John Gerarde (Johnson 1633; Moore 1915). An
excellent history of the genus is available (Moore 1915).

In North America, sago is placed with P. filiformis and P. vaginatus
in the subgenus Coleogeton, in which all leaves are linear or setaceous,
nonfloating, and divided their full length by crosspartitions (Fernald 1950).
Harrison (1949) claims members of this subgenus are, unlike others, water
pollinated. The three coleogetonous species have also been shown to form a
distinct subgroup based on the chemistry of the waters they inhabit (Pip 1987).
In the field, sago can be differentiated from the two other coleogetonous
species by the presence of usually sharp-tipped or gradually pointed leaves
and leaf sheaths that are rather narrow but free at the tips.

Sago has an average of 2n = 78 (70-87) chromosomes (Kalkman and Van Wijk
1984). Analyses of isoenzymes indicated that the species is genetically very
heterogeneous (Hettiarachchi and Triest 1986; Van Wijk et al. 1988). Sago
hybridizes with Potamogeton filiformis (P. x suecicus Richt.)
and P. vaginatus (Hagstrom 1916; Dandy and Taylor 1946; Harrison 1949).
Meriaux (1978) and Van Wijk (1988) reviewed the work of many European taxonomists
who named many varieties or "proles" of sago (dichotomus Wallr., drupaceus
Koch, flabellatus Crep., interceptus Asch., protensus
Wallr., setaceus Mey., scoparius Wallr., vulgaris Cham.
and Schl., and zosteraceus Fries). Both questioned whether these are
simple morphs or truly have value as indicators of specific biotopes or habitat
types. Luther (1951, cited in Van Wijk 1983) also concluded that the different
types of sago were habitat modifications. The varieties interruptus
Asch., pectinatus, and scoparius have been maintained in a recent
European flora, although their taxonomic validity is said to be unclear (Casper
and Krausch 1980, cited in Van Wijk 1988). Van Wijk (1983) found different
morphological and ecological characteristics of annual and perennial P.
pectinatus in the field and in cultured plants and recommended that the
existence of these ecotypes be considered when studying the taxon. Wiegleb
(1978) associated the variety scoparius with HCO3-poor waters
and the variety interruptus with sites polluted with sewage. Recent
work has shown that genetic differentiation does occur in sago and must be
considered along with morphological characters if the taxonomy of the species
is to be clarified (Van Wijk et al. 1988).

Autoecological Classification

Sago is one of only three or four North American species of Potamogeton
that bear starchy underground perennating organs called turions or tubers, although
a few other species have tuberous rootstalks. Sago is generally classified as
a ruderal (capable of occupying mechanically disturbed areas), has multiple
regenerative strategies, and is a stress tolerant, competitive plant that, depending
on exposure to wave action, can alter its allocation of resources to different
reproductive organs (Grime 1979; Kautsky 1987). In growth form, sago is considered
a parvopotamid--that is, a higher aquatic plant rooted in sediment, perennially
submersed except for inflorescences, and possessing long stems and small, mostly
undivided, leaves (Hutchinson 1975). Luxuriant parvopotamid growth results in
dense leaves, branches, and inflorescences in the upper part of the water column,
with much thinner vegetation of stems and widely spaced leaves below; vegetation
density of the upper part increases as water levels drop (Verhoeven 1980a).

Meriaux (1978) reviewed the work of devotees of the Zurich-Montpellier school
of phytosociology (Braun-Blanquet 1932) who placed sago in various orders,
alliances, and associations with other species in this elaborate phytosociological
classification system. Sago was also recognized as a character or dominant
species in several European and Asian associations by Hejny and Husak (1978).
Sago is the most prominent plant in the Potamogeton facies of several
estuarine plant communities in Europe (Kornas et al. 1960) and a faithful
taxon in the class Potamea (den Hartog and Segal 1964) in some wetlands in
India (Zutshi 1975). Sago also is a member of several Chara-, Ruppia-,
and Zannichellia- dominated communities in the Baltic, Mediterranean,
and Eurosiberian regions (Lindner 1978; Verhoeven 1980a; Van Vierssen 1982a).

Sago can be considered a pioneering species, because it quickly inhabits
newly flooded areas (Nelson 1954) and invades shallow waters with relatively
strong wave action (Ozimek and Kowalczewski 1984) or those that are polluted
(Haslam 1978). Sago is one of the first species to colonize areas reclaimed
from the sea (Wolseley 1986). den Hartog (1963) and Van Vierssen (1982a) considered
sago a survivor species that often showed mass development in areas where
the environment became temporarily unsuitable for other species. Davis and
Brinson (1980) placed sago in a group of plants tolerant of, and able to maintain
dominance in, altered ecosystems.

Sago is found in submerged, floating-leaved, and emergent communities. Best
plant development occurs in submerged communities, and the poorest in emergent
communities where sago plants tend to be short in stature (Van der Valk and
Bliss 1971). In general, most other growth forms of hydrophytes, except similar
types such as charids, valisnerids, and ceratophyllids, negatively influence
the environment for parvopotamids, usually because of competition for light
(Hogeweg and Brenkert 1969).

Most submersed macrophytes are sensitive to frost damage (Lohammar 1938).
This, combined with the rapid decomposition of plants in water, causes sago
to usually behave as an annual in shallow waters in temperate climates, with
buried turions the only vegetative structure to survive winter (Lapirov and
Petukhova 1985). However, green sago shoots can be collected under winter
ice, presumably in deeper waters (Hammer and Heseltine 1988). Turions are
perennial diaspores formed underwater and take several weeks or months to
develop. The fruit-like seed (drupelets) can require a stratification period
to germinate well in areas of fairly mild climate. These findings, plus the
observation that sago could not compete well in shallow water against species
that produce seeds (annual diaspores) more quickly, led Van Vierssen and Verhoeven
(1983) to consider sago a species rather intolerant of habitat desiccation.

In mild climates sago can be evergreen (Spence et al. 1979b). Rarely, some
deepwater forms of sago grow perennially from submersed rootstalks and can
also have green shoots that survive winter (Moore 1915). Sago can behave as
an annual by dying under conditions of high salinity and regenerating from
drupelets when salinity decreases (Congdon and McComb 1981). When sago is
compared to Potamogeton nodosus, a species that forms winter buds rather
than turions, both species invest about the same amount of photosynthate in
perennating structures, but sago produces about twice as many propagules (Spencer
and Anderson 1987).

The functional aspects of sago's ability to thrive and survive in a wide
variety of environments have been addressed in detail by Van Wijk (1988) and
will be discussed in later chapters. Van Wijk (1988) points out the confusion
that has resulted from use of the terms annual and perennial to categorize
plant types as well as life-cycle types, and argues that they should only
be used to indicate life cycles of populations without implying a classification
of plant species. Under this system, sago could theoretically be said to have
an annual life cycle with either (1) generative reproduction by seeds or vegetative
reproduction by turions or thickened rhizomes or (2) a perennial life cycle
with vegetative reproduction by whole plants or shoots. Not all of these strategies
have been observed in nature.

In Europe, the Potamogeton pectinatus association is often linked
to brackish water (den Hartog 1963) and inland marshes and depressions affected
by mineral pollution (Meriaux 1978). den Hartog (1981) placed sago with a
small group of plants that share many properties with marine angiosperms but
cannot compete well with them except under special circumstances. He termed
sago a member of the eurysaline group of plants in that they are able to tolerate
waters from fresh to hyperhaline that vary greatly in chemical composition.
These plants are also able to withstand rapid and considerable fluctuations
in salt content of the waters they inhabit., Iversen (1929) included sago
in a group of species restricted to alkaline waters. Lohammar (1938) found
sago in lake waters characterized by both high pH and calcium content. Further
analysis of Lohammar's data by Hutchinson (1975) showed sago to be a eurytopic
species able to tolerate a wide range of nutrient (nitrogen, phosphorus) concentrations.
Moyle (1945) placed sago in an assemblage of hard water species able to withstand
waters high in sulfate ion. Other classifications based on water chemistry
have been proposed by Spence (1967) and Seddon (1972).

Distribution

Unlike most of the Potamogetons, which are interior and northern in global
distribution, sago is nearly cosmopolitan (St. John 1916). The plant occurs
circumboreally to about 70° N (Hulten 1968) and can also be found in South
Africa, South America, South Eurasia, and New Zealand. The species occurs from
sea level to nearly 4,900 m above sea level in high mountains of Venezuela and
Tibet (Ascherson and Graebener 1907, cited in Yeo 1965). Pip (1987) recorded
19 species of Potamogeton at 430 wetland sites distributed throughout
a large area of central North America and found sago second only to P. richardsonii
in frequency of occurrence.
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